The present invention relates to a method of producing rod-shaped articles, particularly such rod-shaped articles which constitute or form part of smokers' products including plain or filter-tipped cigarettes, cigarillos and cigars. More particularly, the invention relates to improvements in a method of producing rod-shaped articles of the type wherein one or more main components are contacted by or otherwise treated or assembled with one or more secondary components to form a rod-shaped intermediate product which is thereupon subdivided into articles of desired length. Typical examples of such articles are filter rod sections wherein the main component consists of a tow of filamentary filter material and the secondary or auxiliary component is a plasticizer or bonding medium which is applied to the tow in a liquid state and is thereupon caused or allowed to set in order to impart to the filter rod sections a desirable resistance to deformation as well as to enhance the smoke-filtering characteristics of such sections.
As a rule, the density of a tow of filamentary filter material which constitutes the main component of filter rod sections or filter elements varies within a wide range. Therefore, it is necessary to regulate the density of the tow prior to formation of a filler which is thereupon wrapped to form a rod and severed to yield filter rod sections of desired length. The filaments of the tow are crimped and the number of crimps per unit length of the tow determines the density of the respective portions of the tow. In the absence of any regulating action, the density of filamentary filter material in filter rod sections would vary not only from increment to increment of a particular filter rod section but also from section to section. Consequently, the resistance which the filter rod sections would offer to the passage of smoke would also vary from filter cigarette to filter cigarette or from filter-tipped cigar to filter-tipped cigar. Such differences in the resistance of filter rod sections to the passage of smoke are highly undesirable.
It is already known to provide a filter rod making machine with means for subjecting the tow to a variable stretching action for the purpose of reducing the difference between the densities of successive increments of the tow. East German Pat. No. 61,974 discloses a detector which scans the tow and produces signals which are utilized for regulation of the stretching action. The detector is a pneumatic detector which determines the resistance to the flow of an air stream across successive increments of the tow or a beta ray detector which includes a source of beta rays at one side of the path of the tow and an ionization chamber which is located opposite the source and furnishes signals indicating the rate at which the beta rays penetrate through the tow. A drawback of pneumatic detectors is that they are not always reliable, i.e., a minor leak in a single one of several conduits which convey the air stream or streams is likely to cause the generation of signals which are not truly indicative of the density of measured portions of the tow. Beta ray detectors are quite expensive and they must be provided with complicated safety devices to protect the attendants.